Thanks, I understand now.

(Dave Martindale) wrote:

writes:
Would someone please fill me in on why we use propane and not natural
gas? I read the Wikipedia article at

http://en.wikipedia.org/wiki/Propane

and I still don't understand the difference. I do know that propane
appliances such as ranges require adjustment to or different burners
for each. Why not eliminate this problem and just use LNG only? In
terms of household heating the only difference then would be that in
one case you have a pipe coming in from the utility and in the other
you have a big tank in the back/front yard and a tanker comes and
fills you up from time to time. Or for your barbeque you go to the
home center and instead of propane you get a tank or a refill of LNG.

Natural gas is mostly methane. It's attractive as a fuel because it's
plentiful, but it's also very difficult to turn into a liquid. Any fuel
that you transport in tanks wants to be a liquid because you can store
many times as much fuel in a given volume tank as a liquid than as a
compressed gas.

Let's back up a bit:
Butane is easy to handle because the amount of pressure needed to keep
it liquid at room temperature can be provided by a plastic container
(e.g. cheap butane lighter) or a very thin aluminum can. Thus, you
can carry a lot of fuel in a small, light tank. But it's relatively
expensive.

Propane requires considerably higher pressure to be liquid at room
temperature, so it has to be stored in heavier steel tanks that can
withstand the pressure. But it *is* liquid in there, so again you can
have quite a bit of fuel inside one small tank.

Methane cannot be practically compressed into a liquid at room
temperature. You can make it liquid by making it very cold; that's how
LNG (liquid natural gas) is produced. But it's got to stay cold to stay
liquid, just like liquid oxygen or nitrogen. It's practical to ship
very large volumes of it in specially-insulated tanks in ships, but it's
not practical to keep that cold in a tank on your car or barbecue.

So when you see a car or bus powered by compressed natural gas (CNG), it
tends to have very large tanks to hold a relatively small amount of gas,
since there is no liquid in the tanks. Propane will power a car for
much longer with a smaller tank. So CNG isn't very popular as a vehicle
fuel.

It isn't practical for house heating or stove or barbecue either. If a
trunk brought you liquid natural gas, you'd need cryogenic storage
(which is expensive) and you would continually lose some fuel to boil
off. If a truck brought you compressed natural gas, you'd need large
tanks, and you'd have to wait for a compressor to re-pressurize them
during fillup.

Natural gas *is* practical if it can be brought to you in gas form at
normal temperature by pipeline. It's just not practical to store or
distribute in a tank except on an enormous scale.

Dave